Materials used for exhaust piping, mufflers, and other exhaust system components consist mainly of ferrous alloys. Aluminium alloys are sometimes used as a coating on ferrous alloys to impart additional corrosion resistance. The choice of exhaust system materials is driven by a number of factors including cost, warranty requirements and legislated and customer demands for long service life. Mild carbon steel was the material of choice for exhaust systems for many decades. An iron oxide coating on the exhaust system protected it from atmospheric corrosion to varying degrees. However, it suffered from poor corrosion resistance when exposed to road salt and exhaust condensate. As a result, exhaust systems made from this material had a very short life if exposed to the environment experienced by many on-road vehicles. The corrosion resistance of carbon steel can be greatly improved through the use of a hot dipped aluminium coating. This is often referred to as aluminised steel.
One particularly important ferrous alloy alloying element is chromium. By adding sufficient chromium, stainless steel is formed. When stainless steel is heated, chromium forms a protective chromium oxide coating that delays further oxidation. A minimum of about 10.5% chromium is usually required to passivate the surface and to classify a material as stainless steel. So long as this oxide layer is stable and continuous, the metal substrate is well protected from corrosion. Since about the mid-1990s, plain carbon and low alloy steels have been replaced by stainless steel as the primary material for exhaust systems. This transition has taken place because of market demands for extended warranties, and because of demands mandated by emission standards. Technologies to meet increasingly stringent emission standards can raise exhaust temperatures which makes the task of meeting strength and durability requirements especially challenging. Emission standards also require that exhaust systems are designed in a manner that facilitates leak-free assembly, installation and operation for the full useful life of the vehicle.
From the early part of the 21st century, commodities including many of the alloying elements used in stainless steel, have experienced wide and rapid price fluctuations. Emission control systems such as actively regenerated diesel particulate filters (DPFs) and urea selective catalytic reduction (SCR) have also created new demands on material properties. Active DPF regeneration can produce exhaust temperatures as high as 800° C. in parts of the exhaust system that would otherwise operate at much lower temperatures. Also, some commonly used stainless steels, such as type 304, have been found to corrode after exposure to urea decomposition products in high temperature environments.